otel_span_attr context if

[fede843]

I am trying to convert several headers into attributes.
The current approach is to go one by one, by using otel_span_attr
otel_span_attr has not if context. I cannot make it conditional. There is no way to keep the span clean, and end up with a bunch of empty attributes.
If otel_span_attr had if context, I could easily go one by one checking if exists. Or maybe there is another idea or proposal to make headers easy to convert other than this.